IntroductionRhodococcus equi is a gram-positive, nonmotile, obligate aerobic, intracellular microorganism. This organism is a facultative pathogen that causes pneumonia in foals. This pathogen also causes ulcerative enterocolitis, osteomyelitis, and septic arthritis in rare cases. R. equi is present in the soil and in horse feces. Foals are thought to become infected when they ingest or breathe in soil, dust, or fecal particles harboring the bacteria within the first few days of life (1,2). Inhalation of aerosolized virulent R. equi from the environment and intracellular replication within alveolar macrophages are essential components of the pathogenesis of R. equi pneumonia in foals (3). Virulence of R. equi is associated with the presence of plasmids of 80-90 kb that encode the 15-17 kDa lipoprotein virulence-associated protein A (VapA) (4). Clinical symptoms typically appear in less than 6 months. However, protection of the newborn foal is most important during the first week (5). In addition, R. equi may cause infections in the lungs of horses (6). R. equi pneumonia significantly impacts the equine industry by causing financial losses because foals that recover from the disease are less likely to race as adults (7). The strategies used to protect against R. equi infection remain unsuccessful in foals. Although the use of a combination of antibiotics, such as rifamycin and erythromycin, is preferred in the treatment of this infection, the therapeutic effects are inconsistent due to bacteria that grow within cells and create granulomatous abscesses. In addition, the long-term use of antibiotics has potential risks, such as the development of resistance to antibiotics. The immunization of mares has been suggested by several researchers to prevent R. equi infection in foals (8-10).In this study, the efficacy of R. equi vaccines was determined in mice for the protection of Arabian foals against R. equi infection. To achieve this aim, 4 inactive vaccine candidates and a live vaccine were prepared using the VapA + K2002 strain of R. equi.